It is well known that filamentous fungi can be used to produce valuable compounds. Due to their glycosylation and secretion capacities, filamentous fungi are preferred hosts for secreting proteins. Secretion is a crucial step in the production of proteins and may become limiting when reaching higher production levels. High-level expression of proteins may compromise protein-folding reactions in the endoplasmic reticulum (ER), causing unfolded or aberrant proteins to accumulate. This stressful situation causes the cell to activate a variety of mechanisms. One adaptive response includes the transcriptional activation of genes encoding ER-resident chaperones and folding catalysts and protein degrading complexes that augment ER folding capacity, as well as translational attenuation to limit further accumulation of unfolded proteins in the ER (Kaufman, 1999; Mori, 2000). This signal transduction cascade is termed the unfolded protein response (UPR). Another means to deal with aberrant ER proteins is through their proteolysis via an ER-Associated Degradation (ERAD) pathway. Thus, UPR and ERAD serve one common goal, which is to decrease stress invoked by accumulation of (aberrant) proteins in the ER, either by decreasing accumulation through increasing solubility of ER localized proteins (UPR), or by increasing degradation of ER localized proteins (ERAD).
UPR and ERAD collaborate to decrease protein accumulation in the ER since it has been shown that increased UPR simultaneously results in increased ERAD (Brodsky, J. L., Werner, E. D., Dubas, M. E., Goeckeler, J. L., Kruse, K. B. and McCracken, A. A. (1999) J. Biol. Chem. 274; 3453-3460).
Recently, WO 01/72783 described a strategy to improve the protein secretion of recombinant eukaryotic cells by manipulating three genes involved in UPR (HAC1, PTC2, IRE1) in eukaryotic cells, to obtain an elevated UPR.
To improve protein secretion capacities of eukaryotic protein production strains it would be highly desirable to avail of strains that possess the capacity to translocate large amounts of a protein of interest through the secretory pathway without accumulating substantial amounts thereof in the ER.
It is an objective of the present invention to provide a method to improve protein secretion capacities of eukaryotic protein producing strains.